A magnetic field measurement system employing a SQUID has been used conventionally to measure a magnetic field (hereinafter referred to as a biomagnetic signal) generated from the brain, heart, or the like of a living body. In the measurement of an extremely weak magnetic field such as a biomagnetic signal, it is necessary to attenuate an external field encroaching in the magnetic field measurement system to the range of 80 dB -100 dB (decibel) or less. An external field is magnetic noise deriving from a power transmission line, a running train, an automobile, or the like and encroaching in the magnetic field measurement system through a commercial power source.
A conventional biomagnetic signal measurement system has been placed inside a magnetically shielded room using a ferromagnetic material such as a permalloy to perform measurement in an environment cut off from an external field. Since the magnetically shielded room using a ferromagnetic material such as a permalloy is high in cost and heavy in weight, medical facilities in which it can be placed are limited. Accordingly, there has been a demand for biomagnetic signal measurement using a simple magnetic shield lighter in weight and smaller in size which allows easy installation of the system even in a narrow place.
Although a magnetic shield having a simpler structure can implement a lower-cost system, it is required to remove an external field from a measured signal for the correction thereof since the simple magnetic shield cannot completely cut off the external field. Various methods have been tried for external field removal or correction, of which the method using a vector magnetometer to measure an external field is common.
A report has been made on a 1-ch magnetocardiograph which is a combination of a magnetometer for biomagnetic signal measurement and a tri-axial vector magnetometer capable of sensing the respective components of an external field in the x-, y-, and z-directions (see Non-Patent Document 1 (Prior Art 1)). In this report, the process of multiplying, by three different factors, signals for the x-, y-, and z-components of the external field sensed by the three magnetometers of the vector magnetometer and subtracting the result of multiplication from an output signal from the magnetometer for biomagnetic signal measurement was performed. In accordance with the method, it is necessary to simultaneously optimize the three factors.
Another report has been made on external field removal in a brain magnetic field measurement system having 165 sensing magnetometers (see Non-Patent Document 2 (Prior Art 2)). In the report, an attempt was made to reduce the influence of an external field by using four tri-axial vector gradiometers for external field measurement which are composed of 12 magnetometers for 153 sensing magnetometers. In accordance with the method also, it is necessary to simultaneously optimize a plurality of factors.
On the other hand, a biomagnetic signal measurement system which measures the normal line component of a biomagnetic signal and obtains the tangent line component thereof from the normal line component has been well known (see, e.g., Patent Document 1).
[Non-Patent Document 1] K. Sakuta et al., Physica C378-381 (2002), 1391-1395
[Non-Patent Document 2] V. Pizzella et al., Proceedings of Biomag, 2000, 939-942
[Patent Document 1]Japanese Laid-Open Patent Publication No. HEI 10-305019